Good evening everyone. So this isn't actually for a helicopter, but if anyone would know, it would be someone who follows this part of the forum. I'm building a 1/8 FMA IA-58 Pucara and would like to incorporate constant speed props. From what I can tell my best best would be to incorperate a tail rotor assembly. Having never worked with RC helicopters, I really don't know what's out there or what I should be looking for. I'm just in the design phase right now, but below are the parameters I'm starting with for the prop assembly:

3-blades
12.5" Prop disk diameter
Turigy G46 420kv Direct Drive (though I am willing to change this as required)
Power will be supplied by 2x 5S (TBD)mAh batteries.
Each hub will need to support approx 100oz of static thrust.

So basically I'm looking for a few things:
-Conceptually, with you guys having worked with tail rotors, does anyone see anything glaringly wrong with the idea?
-If not, does anyone have a recommendation as to a unit that would be acceptable for this use?
-And of course, does anyone have an idea for a better solution?

I'm pretty optimistic that if I can find a suitable unit I should be able to provide a "true turboprop" appearance... super slow start on the esc with the throttle stick simply controlling the blade pitch to adjust power output. Anyways, I appreciate any and all feedback, recommendations and suggestions!

I Googled the plane and took the liberty of attaching a picture of it. Very cool looking bird! While my biggest heli is only a 500 maybe something in the .60-.90 size tail rotor would be a closer size for you but the mechanics of grafting something like that onto a motor/shaft AND make it all fit inside a nacelle could be daunting. Even using a smaller main rotor head setup would be difficult. Hey, maybe it's possible. If you can make it happen I'm going to be following every step of the way! It would be fun to have just for the coolness factor!

I suppose using a 130x main rotor would work. If you could find a way of shutting off as3x on two boards, and perhaps bypassing the boards to run only throttle and pitch, plugging into a separate receiver. You'd be better off using beastx receivers/gyros, and have motors with extended shafts that the swashplates mounts directly onto, along with rotors. This would allow you to use something like 130x rotors no gearing. I am not sure about cancelling out the gyro on the beast x, or making servos work alone.

Joe and thepiper92, thanks. I'm focusing on a tail rotor assembly as opposed to a main rotor hub primarily because I only need 1 degree of freedom with regards to blade pitch... basically it can be mechanically much simpler than a main rotor head. If I went with a main rotor head I would have to use 3 servos that would move everything uniformly as I only want collective input but no cyclic.

I'm not familiar with what as3x is. As long as I get my hands on something that is sufficiently strong (mechanically) I should be able to sort out the rest fairly easily.

Right now I'm looking at something like this (http://www.centuryheli.com/products/...m?prtnm=CN1213)... the more I think about it I may go with a geared option as opposed to direct drive. This is for two reasons. First, it may make it easier (mechanically) to connect the "collective" linkage to a servo. Secondly, it could allow for a smoother "slow start" which I think would really add a nice effect... still in the planning phase so any and all input is welcome

Look at the 450x head, and notice how the grips connect to the swash, as well as two points connecting to the swash further down on the head. You could very easily reverse the two parts to connect to the grips, rather than the swash, and implement a moving part that the arms connect to. It would be more difficult that using a tail assembly...but at $250 for a set, I'd look at a cheaper option

Yeah that's my biggest issue... I don't mind dropping a few bucks on an experiment (which I think this qualifies as)... but that is several planes worth of experimentation, especially when you factor in that the aircraft is a twin. that's why i'm trying to get as much info as I can before I commit to this plan.

The problem I see is finding or making a 3-blade version. This type unit is available in a few sizes (not many) They essentially combine a heli tailrotor with a hollow shaft motor.

You CAN make a hollow shaft version of a larger motor to be able to have more power if needed. The main reason for not finding larger versions made commercially is that these are most often used for indoor models.

It's probably not the highest quality, but certainly pushes it into the realm of affordable experimentation. piper92, i think in your previous post you were explaining a method of locking down the degrees of freedom of the rotor head but i'm not sure i understand.

is there maybe an aftermarket tail rotor part that I could adapt to the axel in the above listed piece in lieu of a multi-DOF swash plate? that may be the simplest option.

Instead of going through all the hassle of using a heli tail rotor, why not consider a constant speed VPP set-up.....:

Or somethig like this:

There are a few three blade prop and drive shaft VVP's available thru HK and Horizon.

You might even take a look at multi-rotor drive system set-ups.........there are several different types of set-ups that use the same principal on quads/hex ect. type crafts......that is if your trying to achieve mechanical prop deflection for the purposes of flight efficiency/performance rather than just for aesthetics...!

A tail rotor drive of a very large heli would just about do it 700 and 800 size helis usually have 6mm tail shafts, which should do the job. I dont know where you would find a three blade hub at that size?

Yeah, the commercially available VPP setups are just too small for this project. According to E-flight the VPP's are "3D CAD engineered for use on 3D models weighing up to 12 ounces (340 g) ready to fly (including VPP system)" That isn't going to cut it for this model and I doubt the chinese clones will fare better. I'm expecting an AUW of near 12 lbs depending on battery selection and some of the final details of this setup.

The inspiration to use tail rotor as an aircraft's constant speed prop came from this guy's youtube video (https://www.youtube.com/watch?v=qtwZi_2otrU) He only uses two blades and they are the stock tail rotor blades, but mechanically the idea is basically the same.

So basically, from everyone's input I'm narrowing it down to two paths:

Given how mechanically straight forward it is (from a conceptual basis), I'm kind of surprised that a company hasn't made a variable pitch prop for the non-3D scale fliers out there.

As of right now, I'm leaning towards Path #2 using an inexpensive 450 side main rotor, specifically the unit pictured below. At $20 I'm sure it's not great quality but the price is amiable to experimentation.

I'd still need to find the axle diameter but maybe if I switch to a geared setup, it will afford me some flexibility in hub/motor pairing. Basically from there, I'd remove the swash plate and replace it with a 3-pronged version of the component pictured below (if I can find one... otherwise I may be looking at fabrication, but conceptually the piece looks mechanically straight forward)

I appreciate all of your thoughts, suggestions and input and continue to look forward to your advice going forward. It should be a pretty slick setup once it's implemented! ...I hope!

Pretty much all these cheap 450 helis are based on the Trex 450, so that being the case the shaft will be 5mm. Typically on the really cheap helis the shaft will also be as soft as cheese!

Thinking about it, I'm not sure these heads will be up to the task to be honest. Consider that these helis are designed to have a head speed of maybe 3000-3300RPM whereas your setup will be spinning at around 13000RPM (assuming batteries are two x 5s in series). Also consider that the centripetal force (the force trying to pull the blades out of the hub) is proportional to RPM squared, so if we forget difference in blade weight the force will be something like 16 times greater than it was designed for. Even allowing for the smaller lighter blades you are still going to be overloading the head by a huge margin.This could be extremely dangerous as a blade being thrown at those sort of revs could be catastrophic.

Excellent point! My calculations are based off of 6.5k-7.5k RPM depending on a given pitch. Still, that is 4x the blade loading... assuming a similar blade weight. Obviously for a whole slew of reasons, I would definitely like to avoid throwing a blade!

Experiment with a smaller scale model (cheaper) and see if the concept will work.
A couple of $30-$40 indoor variable pitch systems and a flat plate foam prototype.

Complexity adds weight and unnecessary weight generally is to be avoided when building an airplane. The fixed pitch props can be produced lighter and cheaper than a variable pitch for the larger models. Also fixed pitch eliminates all of the linkages and the servo(s) for controlling the pitch.

Variable pitch is a cool scale feature but its only visible to others when its sitting n the ground. In flight no one will see the difference.

Excellent point! My calculations are based off of 6.5k-7.5k RPM depending on a given pitch. Still, that is 4x the blade loading... assuming a similar blade weight. Obviously for a whole slew of reasons, I would definitely like to avoid throwing a blade!

If you plan to run the motors on straight 5s battery voltage then you are going to be way down on power using a 420Kv motor. I'd estimate about 350W per motor, 700W total... Not enough to get a model like this off the ground I'm afraid.

It's probably not the highest quality, but certainly pushes it into the realm of affordable experimentation. piper92, i think in your previous post you were explaining a method of locking down the degrees of freedom of the rotor head but i'm not sure i understand.

is there maybe an aftermarket tail rotor part that I could adapt to the axel in the above listed piece in lieu of a multi-DOF swash plate? that may be the simplest option.

I was thinking of turning a non-DFC head into a form of a DFC head, but then removing cyclic. Pizzano's post, the second pick. The head fits on the shaft, and a sleeve goes over this, followed by a screw to secure the head to the shaft. This sleeve has ball links, so I bet it could be modified to control collective with a single servo.

JetPlaneFlyer, I'm not super savvy on the equations governing electrical relationship (in terms of what to expect from a given prop/batt/motor combo) so I've been using www.ecalc.ch and I'm showing that at my maximum planned pitch (9 in) I should be putting out a combined 1010W (mech) and nearly a 1:1 static TW-ratio (of course this would decrease as velocity increases). If need be, I could bump up the motor to the G60-500 which should yields the linked results here. Either way I'll end up making a motor selection once I have a more complete weight buildup.

fhhuber, I get what you're saying about starting with something similar. I guess my logic is that for the price of a "starter" VPP setup I could complete this experimental setup as this plane will be built either way... if it proves to be unfeasibleI can always retrofit the powerplant with a conventional fixed-pitch setup. (in all likelihood, I will finish the layout and construction of the aircraft long before I get this VPP sorted out and will run initial flights with with a fixed pitch setup).

thepiper92, what is a "non-DFC" head vs a "DFC head"? again, I have no actual experience on the rotary-wing side of the hobby.

I dug around and found some rough schematics of his setup and threw them into SolidWorks. The total mass would come in under 100g (not including the add'l servo or the prop blades). It looks promising, I'd just have to talk an acquaintance into milling the parts for me on the cheap.

DFC is direct flight control. Essentially, the 450x has the grip arms running to the swash and arms running from the swash to further down the head from the grips to secure the swash movement. Oddly, smaller helis, like the mcpx and nano cpx have a form of DFC, with a bracket the keeps the grip arms 90 degrees, preventing rotation. DFC is simply more efficient...and less parts, so not sure why it isn't used as the standard. All it needs is the grips to have a mounting area parellel to to the blades, thus keeping the arms 90 degrees with the head, rather than something like the 450x, which has mounting on the grips on a ball joint perpendicular to the grips. In the end, DFC is what a tail rotor is, but with cyclic movement. The good thing is, the price: http://www.hobbyking.com/hobbyking/s...ashplate_.html

With this setup, you have a 5mm shaft I believe, and you should be able to fit it into a motor. As for the milling, you could have a swashplate made or altered removing the pivoting that allows cyclic, and with that, one servo can control the swashplate with only collective.

Watts will change with load. 6x4 will result in a low wattage, while 12x8 may overload the motor or achieve max. From my understanding, low kV is the only way to run bigger props, unless you run less volts...resulting in less watts total, meaning that you are turning a big prop that has no guts. You can also run bigger motors to handle bigger props, which are rated to higher watts, but of course, the bigger the motor, usually the lower the kV, and higher the voltage rating. On my current build of the Hurri, 950kV is the highest I have been able to find, and will be pushing around 600W with the 3s. I may choose to go to 4s, going down a bit in prop size, and maxing the motor out at 800W. From my understanding of it all, if you are pulling too many amps, and overheating the motor, the prop is too big, if you exceed Watt rating then you are going beyond the voltage rated for the motor, or too big of a prop for a given voltage, going beyond what the motor can pull for amps. Kv to me has to do with how many volts you wish to run at, and the prop size you want. If you want a big prop and low voltage, then you will have low Wattage and a higher KV motor. A big prop with higher voltage, then you want lower kV and a achieve more wattage. A small prop, you either want a higher kV or a higher voltage. Bigger motors allow for higher wattage, lower kV, so to achieve that higher wattage, you need a bigger prop increasing amps, or more volts.